(Per)fluoropolyethers having carboxylate end groups (otherwise referred to as carboxylic PFPEs or PFPE carboxylates), in particular monofunctional carboxylic PFPEs, are generally used as surfactants, for the treatment of surfaces or as precursors of PFPEs having other end groups, for example as precursors of amides, esters or alcohols, which in turn can be converted into further derivatives, e.g. acrylate derivatives. For this purpose, it is desirable to use as starting materials as pure as possible carboxylic PFPEs. For example, in the preparation of acrylate derivatives the presence of PFPEs with non-functional (or neutral) end groups might impair the optical properties of the final product. However, certain manufacturing processes for PFPE carboxylates may lead to mono- and/or bifunctional PFPE carboxylates in admixture with non-functional PFPEs and it is therefore necessary to carry out a separation process (otherwise referred to as purification or enrichment process) in order to obtain the PFPE carboxylate with high purity.
Chromatographic techniques for the obtainment of highly pure mono- and bi-functional PFPEs having hydroxy or amino end groups from starting mixtures containing non-functional PFPEs are disclosed, for example, in U.S. Pat. No. 5,262,057 (AUSIMONT S.P.A.), U.S. Pat. No. 5,246,588 (AUSIMONT S.P.A.), U.S. Pat. No. 5,910,614 (AUSIMONT SPA) and U.S. Pat. No. 7,288,682 (SOLVAY SOLEXIS SPA).
In greater detail, U.S. Pat. No. 5,246,588 discloses a process for separating, or enriching, non-functional, monofunctional and bifunctional species in a mixture of PFPEs of the general formula:Z—O—Rf—Y  (I)wherein:Rf=straight or branched perfluoropolyoxyalkylene chainand Z and Y, which can be the same or different from one another, are non-functional groups or functional groups containing —OH, in particular —CF2CH2O(CH2CH2O)sH and —CF2CF2CH2O(CH2CH2O)sH groups wherein s ranges from 0 to 2 (see abstract and col. 1, lines 8-37).
The separation process consists in subjecting the mixture of PFPEs to column chromatography using, as eluents, non-polar fluorinated solvents, either alone or in admixture with polar solvents.
In the examples, the purified mixtures have a functionality of from about 0.8 to about 1.2 and wherein chain Rf has a number average molecular weight (Mn) ranging from about 600 to about 750.
U.S. Pat. No. 5,262,057 discloses a process for separating from one another non-functional, monofunctional and bifunctional species in a mixture of PFPEs of general formula:X—O—Rf—Ywherein:Rf=a perfluoropolyoxyalkylene chain comprising monomeric units of formulas (CF2O), (CF2CF2O), (CF2CF2CF2O) and having an average molecular weight ranging from about 500 to about 10,000andX and Y, equal to or different from one another, are non-functional end groups or functional end groups selected from CF2CH2O(CH2CH2O)sH, —CF2CF2CH2O(CH2CH2O)sH and —CF2CH2NH2, wherein s ranges from 0 to 2 (see col. 1, lines 8-32).
The separation process consists in subjecting the mixture of PFPEs to column chromatography under specific conditions, involving in particular the elution with non polar fluorinated solvents and polar solvents.
U.S. Pat. No. 5,910,614 discloses a process for separating bifunctional PFPEs having hydroxylic terminations from non-functional and monofunctional PFPEs having hydroxylic terminations contained in a mixture of perfluoropolyethers of formula (I):X1—O—Rf—Y  (I)where:Rf is a perfluoropolyoxyalkylene chain having number average molecular weight (Mn) 500-10,000 and wherein the hydroxylic termination is of the type —CFX—CH2OH, with X=F or CF3 (col. 1, lines 4-29).
The process comprises:                adding mixture (I) to a suspension of stationary phase in polar solvents in admixture with low polarity fluorinated solvents, at a defined volume/weight ratio of mixture (I)/stationary phase;        evaporating the solvent until a loose, dry powder is obtained;        a first extraction with low polarity fluorinated solvents;        a second extraction with polar hydrogenated solvents (col. 2, lines 32-52).        
The process can be carried out either continuously or discontinuously (col. 2, lines 23-24).
U.S. Pat. No. 7,288,682 relates to a process for the separation of bifunctional PFPEs having —CH2OH terminations from their mixtures with —CH2OH monofunctional PFPEs; this process comprises at least two adsorption steps of a perfluoropolyether mixture of formula:X1—O—Rf—Y  (I)where:Rf is a perfluoropolyoxyalkylene chain having number average molecular weight 500-10,000 and wherein the hydroxylic termination is of the type —CFX—CH2OH, with X=F or CF3;to an adsorbing solid phase contained in a reactor equipped with stirring and filtration, with separation of a solid phase containing absorbed a PFPE mixture enriched in bifunctional PFPEs from a liquid containing a PFPE mixture having diminished bifunctional species. Thereafter, the solid phase is added with a polar solvent and stirred for a definite time range, then filtration is carried out to separate a liquid phase containing the PFPE having the desired high functionality (col. 1, line 34 to col. 4, line 32).
None of the above patents teaches or suggests that the methods therein disclosed are suitable for purifying mixtures of PFPE carboxylic acids.
A process for preparing highly pure monocarboxylic PFPEs is disclosed in U.S. Pat. No. 8,008,523 (SOLVAY SOLEXIS S.P.A); the process starts from a PFPE mixture of formula:T-O—(RF)z-T′wherein RF is a PFPE chain having molecular weight or number average molecular weight in the range of 180-8,000;z is 0 or 1;and T and T′, equal to or different from one another, are selected from the functional end groups —CF2COF, —CF(CF3)COF, —CF2CF2COF, —CF2CF2 CF2COF, —CF2C(O)CF3, —COF, and the non functional (neutral) end groups —CF3, —CF2CF3, —C3F7, —C4F9, wherein fluorine atoms can be substituted by a certain amount of chlorine atoms.
This mixture is subjected to a first distillation step to obtain a fraction wherein the molecular weight delta (i.e. the difference between the minimum and maximum molecular weight of the components) is lower than 600, partial fluorination of this fraction, esterification and/or hydrolysis of the fluorinated fraction and distillation. At col. 11, lines 21-25, is stated that “ . . . the obtainment by distillation . . . of fractions having a molecular weight delta lower than 600 and the subsequent fluorination of the carbonyl end groups is essential for the obtainment of high purity monofunctional carboxylic compounds.” Furthermore, U.S. Pat. No. 8,008,523 actually teaches that chromatography on a solid support is in fact not suitable for obtaining highly pure PFPE carboxylates. Comparative example 5 of this patent shows that column chromatography of a monocarboxylic PFPE having a molecular weight lower than about 600 on silica gel, using the procedure of the aforementioned U.S. Pat. No. 5,246,588, U.S. Pat. No. 5,262,057 and U.S. Pat. No. 5,910,614 and a mixture of fluorinated solvents (for instance hexafluoroxylene and perfluoroheptane) and polar solvents (for instance alcohols, acetone, ethyl acetate), as suggested by U.S. Pat. No. 5,910,614, does not allow to recover or separate mono- and bicarboxylic PFPEs, because they remain adsorbed on the column silica gel.
However, the above process is difficult and time-consuming, since it needs several steps; furthermore, it can be applied only to PFPEs having narrow molecular weight distributions (delta MW<600); thus, the need is still felt to provide a convenient method for purifying polydispersed PFPE carboxylates from mixtures of non-functional and functional PFPE carboxylates.
EP 2436716 A discloses a process for the separation of mixtures of non-functional PFPEs, mono- and bi-functional PFPE carboxylates. The process comprises subjecting the mixture to absorption treatment on an acid adsorbent which is dispersed in a fluorinated solvent. The adsorbent is then washed with a mixture of a fluorinated solvent and a strong acid in order to remove from the adsorbent the mono-PFPE carboxylate. This process is said to allow reducing the content of bi-functional PFPE carboxylates to a range of 0.1-10% mol with respect to the total moles of mono- and bi-functional PFPE carboxylates.
WOJCIK, L., et al. Separation and determination of perfluorinated carboxylic acids using capillary zone electrophoresis with indirect ohotometric detection. Journal of Chromatography. 2006, vol. 1128, no. 1-2, p. 290-297. discloses a method of separation of perfluorocarboxylic acids by capillary zone electrophoresis. No disclosure or suggestion is made to the separation of PFPEs, nor to batch chromatography processes.
VOOGT, P D, et al. Analytical chemistry of perfuoroalkylated substances. Trends in analytical chemistry. 2006, vol. 25, no. 4, p. 326-342. deals with the analytical chemistry of perfluoroalkylated substances; PFPEs are not specifically mentioned or suggested. Moreover, it appears from FIG. 1 that the sole separation technique therein mentioned is liquid chromatography.